Brexit's two biggest hitters have gone into battle to seize back the referendum initiative from US president Barack Obama by putting immigration at the top of the agenda.
Justice Secretary Michael Gove has warned the UK faces a migration "free for all" unless it breaks away from Brussels as the Leave camp moved to exploit an admission from the Government that EU free movement of labour rules make it harder to curb immigration.
The intervention came as London mayor Boris Johnson came out fighting after being roundly condemned by both sides in the Brexit tussle for his highly personalised attacks on Mr Obama.
The London mayor turned his fire on Prime Minister David Cameron with a scathing assault accusing him of achieving "two thirds of diddly squat" in negotiations with Brussels for a special deal for Britain on immigration and other key demands.
In his Daily Telegraph column Mr Johnson warned the Remain side not to "crow too soon" that the Leave side had been "bombed into submission".
"The Prime Minister asked the EU for reform and got two thirds of diddly squat. That deal shows how contemptuously we will be treated if we remain," Mr Johnson said.
The pro-EU camp was still buoyed by President Obama's insistence it would take up to a decade for a post-Brexit Britain to cut a trade deal with Washington.
The Remain side also warned an admission by Leave campaigner and justice minister Dominic Raab that UK citizens may need visas to visit the continent after a withdrawal from the EU highlighted the negative changes Brexit would bring.
Pressed on The Sunday Politics whether Britons would need visas to travel to France or Germany, Mr Raab said: "Or some other kind of check."
While the pro-Europe camp welcomed the intervention of US presidential front-runner Hillary Clinton on their side, Mr Gove insisted potential new members of the EU such as Turkey and Albania posed a "direct and serious threat" to public services like the NHS, and social harmony.
The Cabinet heavyweight wrote in The Times that the health service faced "unquantifiable strain" unless Britain quits the EU.
Home Secretary Theresa May was making her first major speech of the campaign since backing Mr Cameron's Remain stance after she admitted on the BBC's Andrew Marr Show: "Yes, free movement makes it harder to control immigration, but it does not make it impossible to control immigration."
Ms May, who insisted that border control and immigration were two separate things, also acknowledged migrants would look at the rises in the national living wage when making a decision on whether to come to Britain.
With Mr Obama's interventions dominating the campaign in recent days, Ukip leader Nigel Farage dismissed the president's claim that Britain would have to wait a decade for a trade deal as "rubbish" as he pointed out that Australia managed to complete one in two years.
As the debate grew increasingly bitter, prominent Labour MP Chuka Umunna accused Mr Johnson of being "unfit" to be prime minister after his remarks drawing attention to Mr Obama's Kenyan heritage.
"This man is simply not fit to hold the office that he clearly aspires to, which is the prime minister of our country. This kind of divisive rhetoric is indefensible," the MP told Sky's Murnaghan.
Mr Farage also criticised the London mayor's remarks for "playing the man, not the ball".
Insisting he had not sought to "scare" Britons into rejecting Brexit, Mr Obama moved to explain his comments that Britain would be at the "back of the queue" regarding trade deals.
Mr Obama told the BBC: "My simple point is that it's hard to negotiate trade deals. It takes a long time.
"We wouldn't abandon our efforts to negotiate a trade deal with our largest trading partner, the European market, but rather it could be five years from now, 10 years from now, before we were able to actually get something done."